This expresses the opinion that the perceived flexibility and extensibility designed into the Lisp programming language includes all functionality that is theoretically necessary to write a complex computer program, and that the core implementations of other programming languages often do not supply critical functionality necessary to develop complex programs.

This can be viewed as a commentary on the difficulty of creating an efficient implementation of the large and complex Common Lisp language, or simply a joke about Lisp's eval function. Both Greenspun's rule and Morris's corollary are examples of a characteristic style of hacker humor known as "ha ha only serious".[5]